Systems and methods for controlling release of transferable semiconductor structures

ABSTRACT

The disclosed technology relates generally to methods and systems for controlling the release of micro devices. Prior to transferring micro devices to a destination substrate, a native substrate is formed with micro devices thereon. The micro devices can be distributed over the native substrate and spatially separated from each other by an anchor structure. The anchors are physically connected/secured to the native substrate. Tethers physically secure each micro device to one or more anchors, thereby suspending the micro device above the native substrate. In certain embodiments, single tether designs are used to control the relaxation of built-in stress in releasable structures on a substrate, such as Si (1 0 0). Single tether designs offer, among other things, the added benefit of easier break upon retrieval from native substrate in micro assembly processes. In certain embodiments, narrow tether designs are used to avoid pinning of the undercut etch front.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/245,055, filed Oct. 22, 2015, titled “Systems and Methods forControlling Release of Transferable Semiconductor Structures”, and thisapplication is a Continuation-in-Part of U.S. patent application Ser.No. 14/743,988, filed Jun. 18, 2015, titled “Systems and Methods forControlling Release of Transferable Semiconductor Structures,” whichclaims priority to and the benefit of U.S. Provisional PatentApplication No. 62/014,078, filed Jun. 18, 2014, titled “Systems andMethods for Controlling Release of Transferable SemiconductorStructures” and U.S. Provisional Patent Application No. 62/029,535,filed Jul. 27, 2014, titled “Systems and Methods for Controlling Releaseof Transferable Semiconductor Structures,” all of which are expresslyincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The disclosed technology relates generally to systems and methods forcontrolling the release of micro devices using tethers from a nativesubstrate.

BACKGROUND OF THE INVENTION

The disclosed technology relates generally to the formation oftransferable micro devices. Semiconductor chip or die automated assemblyequipment typically uses vacuum-operated placement heads, such as vacuumgrippers or pick-and-place tools, to pick up and apply devices to asubstrate. It is often difficult to pick up and place ultra-thin orsmall micro devices using this technology. Micro transfer printingpermits the selection and application of these ultra-thin, fragile, orsmall micro devices without causing damage to the micro devicesthemselves.

Micro-structured stamps can be used to pick up micro devices from anative source substrate on which they are formed, transport the microdevices to a non-native destination substrate, and print the microdevices onto the destination substrate. Surface adhesion forces are usedto control the selection and printing of these micro devices onto thedestination substrate. This process can be performed massively inparallel, transferring hundreds to thousands of discrete structures in asingle pick-up and print operation.

Electronically active components can be printed onto the non-nativedestination substrates. For example, these printing techniques can beused to form imaging devices such as flat-panel liquid crystal, LED, orOLED display devices or in digital radiographic plates. In eachinstance, the electronically active components are transferred from anative substrate to a destination substrate (e.g., a non-nativesubstrate used to, for example, form an array of the active micro-devicecomponents). The active components are picked up from the nativesubstrate and transferred to the destination substrate using anelastomer stamp.

Micro transfer printing enables parallel assembly of high-performancesemiconductor micro devices onto virtually any substrate material,including glass, plastics, metals or other semiconductors. Thesubstrates can be transparent or flexible, thereby permitting theproduction of flexible electronic devices. Flexible substrates can beintegrated in a large number of configurations, including configurationsnot possible with brittle silicon-based electronic devices.Additionally, some plastic substrates, for example, are mechanicallyrugged and can be used to provide electronic devices that are lesssusceptible to damage or electronic performance degradation caused bymechanical stress. These materials can be used to fabricate electronicdevices by continuous, high-speed, printing techniques capable ofdistributing electronic devices over large substrate areas at low cost(e.g., roll-to-roll manufacturing). Moreover, these conventional microtransfer-printing techniques can be used to print semiconductor devicesat temperatures compatible with assembly on plastic polymer substrates.In addition, semiconductor materials can be printed onto large areas ofsubstrates thereby enabling continuous, high-speed printing of complexintegrated electrical circuits over large substrate areas. Moreover,fully flexible electronic devices with good electronic performance inflexed or deformed device orientations can be provided to enable a widerange of flexible electronic devices. However, conventional microtransfer printing techniques lack the reproducibility and precisionrequired to efficiently produce electronics with high-density devices atlow cost.

In a conventional micro transfer printing process, prior to transferringmicro devices to a destination substrate, a native source substrate isprovided with a sacrificial layer having sacrificial material and aplurality of micro devices formed at least in part over the sacrificiallayer. The micro devices can be distributed over the native sourcesubstrate and spatially separated from each other by an anchorstructure. The anchors are physically connected or secured to the nativesource substrate and tethers physically secure each micro device to oneor more anchors.

Anchor structures that remain rigidly attached to the substrate andtether structures that join the releasable micro object to the anchorserve to maintain the spatial configuration of the micro objects uponpartial or full separation of the micro object from the bulk substrate.When a transfer stamp picks up the device, the tethers for each devicethat is picked up are broken. Regarding micro transfer printing, see,for example, U.S. Pat. No. 7,982,296, issued Jul. 19, 2011, the contentof which is incorporated herein by reference in its entirety.

However, it has been demonstrated that the release of the activemicro-device components is not reliably controlled and not predictable,leading to inefficiencies, irreproducibilities, and errors. Therefore,there is a need for an improved method and system for efficiently andpredictably controlling the release of semiconductor structures.

SUMMARY OF THE INVENTION

The disclosed technology relates generally to systems and methods forcontrolling the release of micro devices using tethers from a nativesubstrate. The disclosed technology provides reliably controlled andpredictable release of micro-device components from a substrate, leadingto efficiencies, reproducibilities, and less errors duringmicro-assembly. In certain embodiments, the disclosed technologyaccomplishes this using a single, off-center tether per micro-device. Asdescribed herein, this improves the removal of a sacrificial layer. Forexample, in some embodiments, this improves the etching of thesacrificial layer such that the desired area under the micro-device isremoved in its entirety so that the micro-device can be micro-assembled(e.g., the micro-device can be picked up by a elastomer stamp causingthe tether to break).

The disclosed technology relates generally to systems and methods forcontrolling the release of micro objects (devices) from a nativesubstrate using tethers. In some embodiments, micro objects are designedor configured so that their orientation and spatial configuration arepreserved when the micro objects are released from a substrate. Anchorstructures that remain rigidly attached to the native substrate andtether structures that join the releasable micro object to the anchorserve to maintain the spatial configuration of the micro objects uponpartial or full release of the micro object from the substrate. This canbe accomplished by selective removal of a sacrificial layer (e.g., atleast partially underneath the micro object) by undercutting, etching,ablation, or other means. In some embodiments, the sacrificial layer isa portion of the native substrate on which the active components aregrown. This leaves each micro device supported on and connected to thenative substrate by at least one anchor and at least one tether.

In one aspect, the disclosed technology includes an array of microdevices, the array including: a source substrate having a process side;a sacrificial layer comprising silicon (1 0 0) sacrificial material onthe process side of the source substrate; a plurality of releasablemicro objects formed at least in part on the sacrificial layer; aplurality of anchor structures located on the process side of the sourcesubstrate, wherein the anchor structures remain rigidly attached to thesource substrate in the absence of the sacrificial material; and aplurality of tethers, wherein each tether of the plurality of tethersconnects a releasable micro object of the plurality of releasable microobjects to one of the anchor structures and each releasable micro objectis connected to an anchor by a single tether.

In certain embodiments, each of the plurality of tethers is sized andshaped to break when a corresponding micro object is contacted by anelastomer stamp for micro transfer printing from the source substrate toa target substrate, different from the source substrate.

In certain embodiments, the sacrificial material is a portion of thesource substrate.

In certain embodiments, the anchor structure forms a continuousstructure, spanning more than one releasable micro object of theplurality of releasable micro objects in at least one dimension.

In certain embodiments, the anchor structure comprises a plurality ofanchors.

In certain embodiments, each of the plurality of anchors arecharacterized by locally concave or internal corners and each of theplurality of releasable micro objects is locally characterized by convexor external corners.

In certain embodiments, each of the plurality of tethers is a tetherwith a width of 10 μm to 40 μm.

In certain embodiments, each of the plurality of tethers is a tetherwith a narrow shape and a width of 1 μm to 5 μm, 5 μm to 10 μm, 10 μm to15 μm, 15 μm to 20 μm, or 20 μm to 40 μm.

In certain embodiments, each of the tethers comprises one or morenotches that provide a point of fracture when a respective releasablemicro object is moved with respect to the anchor structures.

In certain embodiments, the tethers, the printable structure, and theanchors have geometries such that no rectangle with edges parallel tothe <0 1 1> directions that has an edge passing through the printableobject can be drawn such that all portions of the rectangle are coveredby the anchors, tethers, and printable structure.

In another aspect, the disclosed technology includes a method of makingthin and low-cost wafer-packaged micro-scale devices suitable for microtransfer printing using a (1 0 0) Silicon system, the method including:providing a plurality of micro-scale devices; assembling the micro-scaledevices onto a carrier wafer using micro-assembly techniques, whereinthe carrier wafer comprises Silicon (1 0 0) and a first dielectriclayer; embedding the assembled micro-scale devices within a second layerof dielectric; and patterning the first and second dielectric layers todefine a perimeter of each of the micro-scale devices with anchors andtethers that preserve the spatial configuration of the micro-scaledevices with respect to the carrier wafer when the micro-scale devicesare moved with respect to the carrier wafer, thereby providing awafer-level thin wafer package having micro-scale devices suitable formicro transfer printing to other substrates.

In certain embodiments, forming pad structures on at least one of thetop or bottom surfaces of the micro-scale devices, thereby forming asurface-mountable device.

In certain embodiments, the micro scale devices each comprises anintegrated circuit interconnected with at least two sensors and anantenna produced using the same wafer-level metallization.

In certain embodiments, the method includes micro transfer printing themicro-scale devices onto a reeled tape; and applying the micro-scaledevices to a destination substrate using a tape-fed high-speed chipshooter.

In certain embodiments, the method includes pre-molding the micro-scaledevices using a wafer-fed die-attach tool, thereby formingpackage-on-lead-frames.

In certain embodiments, the method includes forming package-in-packagedevices of the micro-scale devices using a wafer-fed die-attach tool.

In certain embodiments, the method includes forming wafer-level-packagesfrom the micro-scale devices using a wafer-fed die-attach tool or awafer-fed micro-transfer printer.

In another aspect, the disclosed technology includes a method offabricating a printable component array, the method including: forming asacrificial layer comprising silicon (1 0 0) sacrificial material on aprocess side of a source substrate; forming a plurality of releasablemicro objects at least in part on the sacrificial layer; forming ananchor structure on the source substrate that remains rigidly attachedto the source substrate in the absence of the sacrificial material; andforming a plurality of tethers, wherein each tether of the plurality oftethers connects a releasable micro object of the plurality ofreleasable micro objects to one of the anchor structures, and eachtether of the plurality of tethers is located on an off-center,anchor-facing edge of the respective releasable micro object of theplurality of releasable micro objects, and each tether is shaped tofracture in response to movement of the releasable micro object, so thatin the absence of the sacrificial material: the releasable micro objectsmove with respect to the anchor structures; the tethers deform and aremechanically stressed; and each tether of the plurality of tethersremains rigidly attached to both a respective anchor and a respectivereleasable micro object of the plurality of micro objects, therebypreserving the spatial configuration of the plurality of releasablemicro objects with respect to the source substrate; and removing atleast a portion of the sacrificial material underneath the plurality ofreleasable micro objects so that the releasable micro objects move withrespect to the anchor structures and the tethers deform and aremechanically stressed.

In certain embodiments, the removal process progresses rapidly under theplurality of releasable micro objects relative to the rate at which theanchor structure is released.

In certain embodiments, each tether of the plurality of tethers has anarrow shape with a width of 10 μm to 40 μm, thereby inhibiting theformation of locally-concave or internal corners.

In certain embodiments, the sacrificial layer has an anisotropic crystalstructure for which the removal process progresses faster in somedirections and slower in other directions.

In certain embodiments, each tether of the plurality of tetherscomprises one or more notches, the notch providing a point of fracturefor a releasable micro object when the releasable micro object is moved.

In certain embodiments, the removal process reaches completion at areasnear a given tether.

In another aspect, the disclosed technology includes a method offabricating a plurality of transferable micro objects, the methodincluding: forming a sacrificial layer comprising silicon (1 0 0)sacrificial material on a process side of a source substrate; forming aplurality of releasable micro objects (e.g., printable electroniccomponents, printable active component, micro devices, micro-scaledevices) at least in part on the sacrificial layer; applying a polymerlayer (e.g., photoresist materials, photodefinable materials) over theplurality of releasable micro objects and at least a portion of thesource substrate, wherein the polymer layer encapsulates the pluralityof releasable micro objects (e.g., the portion of the plurality ofreleasable micro objects other than portions in contact with thesacrificial layer); treating the polymer layer to form: (i) a pluralityof anchor structures on the source substrate for the plurality oftransferable micro objects, (ii) at least one tether between eachtransferable micro object and a pre-determined anchor structure of theplurality of anchor structures, and (iii) for each of the releasablemicro objects, a port of ingress in the polymer layer to a portion ofthe sacrificial layer underneath a respective transferable micro object;and removing (e.g., by an undercutting etch or ablation) at least aportion of the sacrificial layer underneath the plurality of releasablemicro objects.

In certain embodiments, the one or more anchor structures remainsrigidly attached to the substrate when the transferable micro object ismoved.

In certain embodiments, at least a portion of the sacrificial layerunderneath the plurality of releasable micro objects causes each of theplurality of releasable micro objects to move and apply stress on arespective subset of the plurality of tethers.

In certain embodiments, the respective subset of the plurality oftethers comprises a single tether.

In certain embodiments, the method includes transferring the releasablemicro objects and removing at least a portion of the polymer from themicro objects.

In certain embodiments, removing at least a portion of the polymer fromthe micro objects comprises at least one of dissolution, etching, andashing of the polymer.

In certain embodiments, the method includes transferring the releasablemicro objects by contacting at least a portion of a printing stamp to acorresponding portion of the polymer.

In certain embodiments, the tethers, the printable structure, and theanchors have geometries such that no rectangle with edges parallel tothe <0 1 1> directions that has an edge passing through the printableobject can be drawn such that all portions of the rectangle are coveredby the anchors, tethers, and printable structure.

In another aspect, the disclosed technology includes an array of microdevices, the array including: a source substrate having a process side;a sacrificial layer comprising silicon (1 0 0) sacrificial material onthe process side of the source substrate; a plurality of releasablemicro objects formed at least in part on the sacrificial layer; aplurality of anchor structures located on the process side of the sourcesubstrate, wherein the anchor structures remain rigidly attached to thesource substrate in the absence of the sacrificial material; and aplurality of tethers, wherein each tether of the plurality of tethersconnects a releasable micro object of the plurality of releasable microobjects to one of the anchor structures and each tether of the pluralityof tethers is located on an off-center, anchor-facing edge of therespective releasable micro object of the plurality of releasable microobjects, so that in the absence of the sacrificial material thereleasable micro objects move with respect to the anchor structures andthe tethers deform and are mechanically stressed.

In certain embodiments, each of the plurality of tethers is sized andshaped to break when a corresponding micro object is contacted by anelastomer stamp for micro transfer printing from the source substrate toa target substrate, different from the source substrate.

In certain embodiments, the sacrificial material is a portion of thesource substrate.

In certain embodiments, the anchor structure forms a continuousstructure, spanning more than one releasable micro object of theplurality of releasable micro objects in at least one dimension.

In certain embodiments, the anchor structure comprises a plurality ofanchors.

In certain embodiments, each of the plurality of anchors arecharacterized by locally concave or internal corners and each of theplurality of releasable micro objects is locally characterized by convexor external corners.

In certain embodiments, each of the plurality of tethers is a tetherwith a width of 10 μm to 40 μm.

In certain embodiments, each of the plurality of tethers is a tetherwith a narrow shape and a width of 1 μm to 5 μm, 5 μm to 10 μm, 10 μm to15 μm, 15 μm to 20 μm, or 20 μm to 40 μm.

In certain embodiments, each of the tethers comprises one or morenotches that provide a point of fracture when a respective releasablemicro object is moved with respect to the anchor structures.

In certain embodiments, the tethers, the printable structure, and theanchors have geometries such that no rectangle with edges parallel tothe <0 1 1> directions that has an edge passing through the printableobject can be drawn such that all portions of the rectangle are coveredby the anchors, tethers, and printable structure.

In another aspect, the invention is directed to an array of microdevices, the array comprising: a source substrate having a process side;a sacrificial layer comprising sacrificial material on the process sideof the source substrate; a plurality of releasable micro objects formedat least in part on the sacrificial layer; a plurality of anchorstructures located on the process side of the source substrate, whereinthe anchor structures remain rigidly attached to the source substrate inthe absence of the sacrificial material; and a plurality of tethers,wherein each tether of the plurality of tethers connects a releasablemicro object of the plurality of releasable micro objects to one of theanchor structures and each tether of the plurality of tethers is locatedon an off-center, anchor-facing edge of the respective releasable microobject of the plurality of releasable micro objects, so that in theabsence of the sacrificial material the releasable micro objects movewith respect to the anchor structures and the tethers deform and aremechanically stressed.

In certain embodiments, each of the plurality of tethers is sized andshaped to break when a corresponding micro object is contacted by anelastomer stamp for micro transfer printing from the source substrate toa target substrate, different from the source substrate.

In certain embodiments, the sacrificial material is a portion of thesource substrate. In certain embodiments, the anchor structure forms acontinuous structure, spanning more than one releasable micro object ofthe plurality of releasable micro objects in at least one dimension. Incertain embodiments, the anchor structure comprises a plurality ofanchors.

In certain embodiments, each of the plurality of anchors arecharacterized by locally concave or internal corners and each of theplurality of releasable micro objects is locally characterized by convexor external corners.

In certain embodiments, each of the plurality of tethers is a tetherwith a width of 10 μm to 40 μm. In certain embodiments, each of theplurality of tethers is a tether with a narrow shape and a width of 1 μmto 5 μm, 5 μm to 10 μm, 10 μm to 15 μm, 15 μm to 20 μm, or 20 μm to 40μm.

In certain embodiments, the sacrificial layer has an anisotropic crystalstructure.

In certain embodiments, the sacrificial layer comprises a materialselected from the group consisting of Silicon (1 0 0), InAlP, InP, GaAs,InGaAs, AlGaAs, GaSb, GaAlSb, AlSb, InSb, InGaAlSbAs, InAlSb, and InGaP.In certain embodiments, the sacrificial layer comprises Silicon (1 0 0).

In certain embodiments, each of the tethers comprises one or morenotches that provide a point of fracture when a respective releasablemicro object is moved with respect to the anchor structures.

In certain embodiments, the source substrate comprises a member selectedfrom the group consisting of Silicon (1 0 0), silicon, indium phosphide,gallium arsenide, and sapphire.

In certain embodiments, each of the tethers has an aspect ratio ofgreater than 1.

In another aspect, the invention is directed to a method of making thinand low-cost wafer-packaged micro-scale devices suitable for microtransfer printing using a (1 0 0) Silicon system, the method comprising:providing a plurality of micro-scale devices; assembling the micro-scaledevices onto a carrier wafer using micro-assembly techniques, whereinthe carrier wafer comprises Silicon (1 0 0) and a first dielectriclayer; embedding the assembled micro-scale devices within a second layerof dielectric; patterning the first and second dielectric layers todefine a perimeter of each of the micro-scale devices with anchors andtethers that preserve the spatial configuration of the micro-scaledevices with respect to the carrier wafer when the micro-scale devicesare moved with respect to the carrier wafer, thereby providing awafer-level thin wafer package having micro-scale devices suitable formicro transfer printing to other substrates.

In certain embodiments, the method further comprises: forming padstructures on at least one of the top or bottom surfaces of themicro-scale devices, thereby forming a surface-mountable device.

In certain embodiments, the micro scale devices each comprises anintegrated circuit interconnected with at least two sensors and anantenna produced using the same wafer-level metallization. In certainembodiments, the method further comprises: micro transfer printing themicro-scale devices onto a reeled tape; and applying the micro-scaledevices to a destination substrate using a tape-fed high-speed chipshooter.

In certain embodiments, the method further comprises: pre-molding themicro-scale devices using a wafer-fed die-attach tool, thereby formingpackage-on-lead-frames.

In certain embodiments, the method further comprises: formingpackage-in-package devices of the micro-scale devices using a wafer-feddie-attach tool.

In certain embodiments, the method further comprises: formingwafer-level-packages from the micro-scale devices using a wafer-feddie-attach tool or a wafer-fed micro-transfer printer.

In another aspect, the invention is directed to a method of fabricatinga printable component array, the method comprising: forming asacrificial layer comprising sacrificial material on a process side of asource substrate; forming a plurality of releasable micro objects atleast in part on the sacrificial layer; forming an anchor structure onthe source substrate that remains rigidly attached to the sourcesubstrate in the absence of the sacrificial material; forming aplurality of tethers, wherein each tether of the plurality of tethersconnects a releasable micro object of the plurality of releasable microobjects to one of the anchor structures, each tether of the plurality oftethers is located on an off-center, anchor-facing edge of therespective releasable micro object of the plurality of releasable microobjects, and each tether is shaped to fracture in response to movementof the releasable micro object, so that in the absence of thesacrificial material: the releasable micro objects move with respect tothe anchor structures; the tethers deform and are mechanically stressed;and each tether of the plurality of tethers remains rigidly attached toboth a respective anchor and a respective releasable micro object of theplurality of micro objects, thereby preserving the spatial configurationof the plurality of releasable micro objects with respect to the sourcesubstrate; removing at least a portion of the sacrificial materialunderneath the plurality of releasable micro objects so that thereleasable micro objects move with respect to the anchor structures andthe tethers deform and are mechanically stressed.

In certain embodiments, the removal process progresses rapidly under theplurality of releasable micro objects relative to the rate at which theanchor structure is released. In certain embodiments, each tether of theplurality of tethers has a narrow shape with a width of 10 μm to 40 μm,thereby inhibiting the formation of locally-concave or internal corners.

In certain embodiments, the sacrificial layer has an anisotropic crystalstructure for which the removal process progresses faster in somedirections and slower in other directions.

In certain embodiments, each tether of the plurality of tetherscomprises one or more notches, the notch providing a point of fracturefor a releasable micro object when the releasable micro object is moved.

In certain embodiments, the removal process reaches completion at areasnear a given tether.

In certain embodiments, the source substrate is a material selected fromthe group consisting of Silicon (1 0 0), silicon, indium phosphide,gallium arsenide, and sapphire.

In certain embodiments, each of the plurality of tethers has an aspectratio of less than 1.

In another aspect, the invention is directed to a method of fabricatinga plurality of transferable micro objects, comprising: forming asacrificial layer comprising sacrificial material on a process side of asource substrate; forming a plurality of releasable micro objects (e.g.,printable electronic components, printable active component, microdevices, micro-scale devices) at least in part on the sacrificial layer;applying a polymer layer (e.g., photoresist materials, photodefinablematerials) over the plurality of releasable micro objects and at least aportion of the source substrate, wherein the polymer layer encapsulatesthe plurality of releasable micro objects (e.g., the portion of theplurality of releasable micro objects other than portions in contactwith the sacrificial layer); treating the polymer layer to form: (i) aplurality of anchor structures on the source substrate for the pluralityof transferable micro objects, (ii) at least one tether between eachtransferable micro object and a pre-determined anchor structure of theplurality of anchor structures, and (iii) for each of the releasablemicro objects, a port of ingress in the polymer layer to a portion ofthe sacrificial layer underneath a respective transferable micro object;and removing (e.g., by an undercutting etch or ablation) at least aportion of the sacrificial layer underneath the plurality of releasablemicro objects.

In certain embodiments, the one or more anchor structures remainsrigidly attached to the substrate when the transferable micro object ismoved.

In certain embodiments, at least a portion of the sacrificial layerunderneath the plurality of releasable micro objects causes each of theplurality of releasable micro objects to move and apply stress on arespective subset of the plurality of tethers.

In certain embodiments, the respective subset of the plurality oftethers comprises a single tether.

In certain embodiments, the method further comprises transferring thereleasable micro objects and removing at least a portion of the polymerfrom the micro objects. In certain embodiments, the method furthercomprises removing at least a portion of the polymer from the microobjects comprises at least one of dissolution, etching, and ashing ofthe polymer. In certain embodiments, the method further comprisestransferring the releasable micro objects by contacting at least aportion of a printing stamp to a corresponding portion of the polymer.

In certain embodiments, single-tether designs are used to control therelaxation of built-in stress in releasable structures on a substrate,such as Si (1 0 0). Single-tether designs offer, among other things, theadded benefit of easier breaking upon removal from a native substrate inmicro-assembly processes. In certain embodiments, narrow-tether designs(e.g., tethers with a width of 1 μm to 5 μm, 5 μm to 10 μm, 10 μm to 15μm, 15 μm to 20 μm, or 20 μm to 40 μm) are used to avoid pinning of anundercutting etch front when etching the sacrificial material from thesacrificial layer.

The disclosed technology, in certain embodiments, includes a method offabricating a printable component array. The method can include forminga sacrificial layer having sacrificial material on a process side of asource substrate; forming a plurality of releasable micro objects (e.g.,printable electronic components, printable active component) at least inpart on the sacrificial material; forming an anchor structure on thesource substrate that remains rigidly attached to the substrate in theabsence of the sacrificial material when the sacrificial material is atleast partially removed; performing a removal process (e.g., etch orablation undercutting the micro objects) to remove at least a portion ofthe sacrificial material from the sacrificial layer underneath theplurality of releasable micro objects, thereby causing each of theplurality of releasable micro objects to relax and apply mechanicalstress on a respective subset (e.g., a single tether) of a plurality oftethers when the micro objects move with respect to the anchorstructures so that the tethers deform and are mechanically stressed, forexample in response to contact with an elastomer stamp.

In certain embodiments, a single tether of the plurality of tethersphysically secures each of the plurality of micro objects to the anchorstructure, thereby controlling the relaxation of built-in stress in theplurality of releasable micro objects after the undercut removal processis performed. Each tether of the plurality of tethers can be shaped tofracture responsive to pressure applied thereto (e.g., duringtransfer/printing). Each tether of the plurality of tethers can remainrigidly attached to both a respective anchor and a respective releasablemicro object of the plurality of micro objects, thereby preserving thespatial configuration of remaining micro objects upon transfer of atleast a portion of the plurality of micro objects from the sourcesubstrate (e.g., via micro transfer printing).

The disclosed technology, in certain embodiments, includes an array ofmicro objects. The array can include a source substrate; a sacrificiallayer on a process side of the source substrate; a plurality ofreleasable micro objects (e.g., printable electronic components,printable active components) formed at least in part on top of thesacrificial layer material; an anchor structure located on the processside of the source substrate, wherein the anchor structure remainsrigidly attached to the substrate when the sacrificial layer is removed,thereby preserving the spatial configuration of the plurality ofreleasable micro objects with respect to the source substrate; aplurality of tethers, wherein a subset (e.g., a single tether) of theplurality of tethers connects each of the plurality of releasable microobjects to the anchor structure, thereby controlling relaxation ofbuilt-in stress in the plurality of releasable micro objects after aremoval process removes at least a portion of the sacrificial layerunderneath the plurality of releasable micro objects (e.g., anundercutting etch or ablation).

The anchor structure can remain rigidly attached to the substrate whenthe sacrificial layer is removed by the removal process (e.g., anundercutting etch or ablation), thereby causing the plurality ofreleasable micro objects to move and apply additional stress on at leasta portion of the plurality of tethers, each of which connects one of theplurality of releasable micro objects to the substrate via the anchorstructure after the sacrificial material removal is performed.

The disclosed technology, in certain embodiments, includes a method ofmaking transfer-ready integrated circuits using a silicon-on-insulatorsource wafer with a (1 0 0) handle wafer. The method can include forminga silicon-on-insulator wafer comprising a handle wafer with a (1 0 0)orientation; forming a plurality of transfer-ready integrated circuitson the silicon-on-insulator wafer; patterning the device silicon layer(e.g., thereby physically defining the shape of the integrate circuits);patterning and etching through dielectric layers around the perimeter ofeach of the transfer-ready integrated circuits, thereby defining aplurality of micro devices; and depositing an inorganic dielectric topassivate the exposed device silicon.

The disclosed technology, in certain embodiments, includes a method ofgenerating thin and low-cost wafer-packaged micro-scale devices suitablefor micro transfer printing using a (1 0 0) system. The method caninclude providing a plurality of micro-scale devices and assembling themicro-scale devices onto a carrier wafer using micro-assemblytechniques, wherein the carrier wafer comprises (1 0 0) oriented siliconand a first dielectric layer (e.g., an organic material, for examplepolyimide, or an inorganic material, for example silicon dioxide);embedding the assembled micro devices within a second layer ofdielectric (e.g., an organic material, for example a spin-coatedpolyimide, or a deposited inorganic material); patterning the first andsecond dielectric layers, to define a perimeter of the micro-scaledevices with anchors and tethers that preserve the spatial configurationof the micro-scale devices with respect to the carrier wafer when themicro-scale devices are moved with respect to the carrier wafer, therebyproviding a wafer-level thin wafer package having micro-scale devicessuitable for micro transfer printing to other substrates (e.g., usingvacuum collets or using a transfer element that physically contacts thedevices).

The disclosed technology, in certain embodiments, includes a method offabricating a plurality of transferable micro objects. The methodincludes forming a sacrificial layer including sacrificial material on aprocess side of a source substrate; forming a plurality of releasablemicro objects (e.g., printable electronic components, printable activecomponent, micro devices, micro-scale devices) at least in part on thesacrificial layer; applying a polymer layer (e.g., photoresistmaterials, photodefinable materials) over the plurality of releasablemicro objects and at least a portion of the source substrate, whereinthe polymer layer encapsulates the plurality of releasable micro objects(e.g., the portion of the plurality of releasable micro objects otherthan portions in contact with the sacrificial layer); treating thepolymer layer to form: (i) a plurality of anchor structures on thesource substrate for the plurality of transferable micro objects, theone or more anchor structures remaining rigidly attached to thesubstrate when the sacrificial layer of the transferable micro object ismoved, (ii) at least one tether between each transferable micro objectand a pre-determined anchor structure of the plurality of anchorstructures, and (iii) for each of the releasable micro objects, a portof ingress in the polymer layer to a portion of the sacrificial layerunderneath a respective transferable micro object; and performing aremoval process (e.g., an undercutting etch or ablation) to remove atleast a portion of the sacrificial layer underneath the plurality ofreleasable micro objects (e.g., thereby causing each of the plurality ofreleasable micro objects to move and apply stress on a respective subset(e.g., a single tether) of the plurality of tethers).

The disclosed technology, in certain embodiments, allows for a singlepolymer layer (e.g., a photoresist or photo-sensitive material) to beemployed during the fabrication of the array of micro devices. Thesingle layer (e.g., encapsulation structure) can serve as an anchorstructure, a tether structure, or an encapsulation structure or allthree. The encapsulation structure, in some embodiments, serves toprotect the array of micro devices (including any chemically-sensitivelayers thereof) during the device fabrication and micro-printing processand to provide a contact and adhesion interface to the transferringelements employed in the micro-printing process. The encapsulationstructure can provide electrical insulation and protection fromcontaminants, moisture, and oxidizers. The layer can further enableelectrical contacts on the bottom, top, or side of a printable object.

In some embodiments, the single polymer layer improves the density ofthe printable, micro devices for a given wafer. The supporting,non-printable structures (e.g., the anchor and tether) can be fabricatedsuch that distance between adjacent printable structures is small (e.g.,less than half the width of the printable structures).

In some embodiments, polymers (e.g., photoresist or photo-sensitivematerials) have fracture characteristics that are desirable as a tether,such as sufficient stiffness to maintain the spatial configurations ofprintable structures during a release process. The polymer that formsthe anchoring, tethering, or encapsulation can also be selectivelyremoved from the printable objects and the target substrate aftertransfer by dissolution, etching, ashing or other processes. In someembodiments, the polymer also has sufficient adhesion to serve as acontact interface with a transfer element of the micro-printing systemto be picked up from native substrate and can be treated to serve as aninterface for separation between the transfer element and the microdevices once the micro devices have been transferred to a destinationsubstrate. Thus, in an embodiment, the releasable micro objects aretransferred by contacting at least a portion of a printing stamp to acorresponding portion of the polymer.

The disclosed technology, in certain embodiments, allows for adielectric layer (e.g., a silicon dioxide or silicon nitride) to beemployed during the fabrication of the array of micro devices. Thedielectric layer can serve as an anchor structure, a tether structure,or an encapsulation structure or all three. The encapsulation structure,in some embodiments, serves to protect the array of micro devices(including any chemically-sensitive layers thereof) during the devicefabrication and micro-printing process and to provide a contact andadhesion interface to the transferring elements employed in themicro-printing process. The encapsulation structure can provideelectrical insulation and protection from contaminants, moisture, andoxidizers. The layer can further enable electrical contacts on thebottom, top, or side of a printable object.

In some embodiments, the dielectric layer improves the density of theprintable, micro devices for a given wafer. The supporting,non-printable structures (e.g., the anchor and tether) can be fabricatedsuch that distance between adjacent printable structures is small (e.g.,less than half the width of the printable structures).

In some embodiments, dielectric layers have fracture characteristicsthat are desirable as a tether, such as sufficient stiffness to maintainthe spatial configurations of printable structures during a releaseprocess. The dielectric that forms the anchoring, tethering, orencapsulation can also be selectively removed from the printable objectsand the target substrate after transfer by dissolution, etching, orother processes. In some embodiments, the dielectric layer also hassufficient adhesion to serve as a contact interface with a transferelement of the micro-printing system to be picked up from nativesubstrate and can be treated to serve as an interface for separationbetween the transfer element and the micro devices once the microdevices have been transferred to a destination substrate. Thus, in anembodiment, the releasable micro objects are transferred by contactingat least a portion of a printing stamp to a corresponding portion of thedielectric material.

In another aspect, the disclosed technology includes an array of microdevices, the array including: a source substrate having a process side;a sacrificial layer comprising sacrificial material on the process sideof the source substrate; a plurality of releasable micro objects formedat least in part on the sacrificial layer; a plurality of anchorstructures located on the process side of the source substrate, whereinthe anchor structures remain rigidly attached to the source substrate inthe absence of the sacrificial material; and a plurality of tethers,wherein each tether of the plurality of tethers connects a releasablemicro object of the plurality of releasable micro objects to one of theanchor structures and each tether of the plurality of tethers is locatedon an off-center, anchor-facing edge of the respective releasable microobject of the plurality of releasable micro objects, so that in theabsence of the sacrificial material the releasable micro objects movewith respect to the anchor structures and the tethers deform and aremechanically stressed.

In certain embodiments, each of the plurality of tethers is sized andshaped to break when a corresponding micro object is contacted by anelastomer stamp for micro transfer printing from the source substrate toa target substrate, different from the source substrate.

In certain embodiments, the sacrificial material is a portion of thesource substrate.

In certain embodiments, the anchor structure forms a continuousstructure, spanning more than one releasable micro object of theplurality of releasable micro objects in at least one dimension.

In certain embodiments, the anchor structure comprises a plurality ofanchors.

In certain embodiments, each of the plurality of anchors arecharacterized by locally concave or internal corners and each of theplurality of releasable micro objects is locally characterized by convexor external corners.

In certain embodiments, each of the plurality of tethers is a tetherwith a width of 10 μm to 40 μm.

In certain embodiments, each of the plurality of tethers is a tetherwith a narrow shape and a width of 1 μm to 5 μm, 5 μm to 10 μm, 10 μm to15 μm, 15 μm to 20 μm, or 20 μm to 40 μm.

In certain embodiments, the sacrificial layer has an anisotropic crystalstructure.

In certain embodiments, the sacrificial layer includes a materialselected from the group consisting of Silicon (1 0 0), InAlP, InP, GaAs,InGaAs, AlGaAs, GaSb, GaAlSb, AlSb, InSb, InGaAlSbAs, InAlSb, and InGaP.

In certain embodiments, the sacrificial layer comprises Silicon (1 0 0).

In certain embodiments, each of the tethers comprises one or morenotches that provide a point of fracture when a respective releasablemicro object is moved with respect to the anchor structures.

In certain embodiments, the source substrate comprises a member selectedfrom the group consisting of Silicon (1 0 0), silicon, indium phosphide,gallium arsenide, and sapphire.

In certain embodiments, each of the tethers has an aspect ratio ofgreater than 1.

In certain embodiments, the sacrificial layer comprises InAlP.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and other objects, aspects, features, and advantages ofthe present disclosure will become more apparent and better understoodby referring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is an illustration of an example native substrate with microdevices formed thereon;

FIG. 2 is an illustration of the process of undercut etching over time;

FIG. 3 is an illustration of the process of undercut etching atransferable semiconductor with a single tether;

FIG. 4 is an illustration of the process of undercut etching atransferable semiconductor with a single tether placed off-center;

FIGS. 5A and 5B are illustrations of an example structure with fourtethers per releasable structure;

FIGS. 6A and 6B are example illustrations of a tether specific to (11 1) silicon;

FIGS. 7A and 7B illustrate additional designs for crystallographicselectivity in removing a sacrificial layer from underneath thereleasable object and less underneath the anchoring structure;

FIG. 8 is an illustration of an example notch implemented in a tether;

FIG. 9 is a flowchart of an example method for providing ultra-thinlow-cost packaged devices using (1 1 1) silicon;

FIGS. 10A through 10D illustrate a processor for making transfer-readyintegrated circuits using a silicon-on-insulator wafer with a (1 1 1)handle wafer;

FIGS. 11A through 11C illustrate an example process of assembling alow-cost carrier wafer using micro-assembly techniques using elastomerstamp micro-transfer-printing to pick-up and place the thin microdevices onto another carrier wafer;

FIGS. 12A through 12C illustrate an example method for generating atouch, thin, and low-cost packaged device using the (1 1 1) system;

FIG. 13 is an illustration of an example of a substrate with releaseddielectric structures thereon;

FIG. 14 is an illustration of example adaptable assembly routes withwafer-level (1 1 1) silicon enabled packages;

FIG. 15 is an illustration of an example system in a package using the(1 1 1) release system;

FIG. 16 is an illustration of an example showing a releasable microobject with anchors, tethers and encapsulation;

FIG. 17 is an illustration of release and removal showing an examplereleasable micro object;

FIG. 18 is an illustration of crystal orientation for semiconductordevices;

FIGS. 19 and 20 are illustrations of an example tether design;

FIGS. 21A and 21B are cross sections of an example tether design with asilicon crystalline structure orientation;

FIGS. 22 through 24 are illustrations of tether designs according toembodiments of the present invention;

FIG. 25 is an illustrative counterexample of tether structures andsilicon crystalline orientation that do not etch effectively;

FIG. 26 illustrates an embodiment with tethers attached to the cornersof the releasable micro object in accordance with an embodiment of theinvention;

FIG. 27 illustrates corner tethers that also have an overlappingdistance;

FIG. 28 illustrates a first pair of tethers that are offset in they-direction and do not overlap in the offset direction as shown withdistance and a second pair of tethers that are offset in the x-directionand do not overlap in the offset direction as shown with distance;

FIG. 29 illustrates a configuration that will have difficulty releasing;

FIG. 30 illustrates a pair of tethers that are offset in both thex-direction and y-direction and do not overlap in either offsetdirection; and

FIGS. 31A and 31B are illustrations of examples of tether and anchorarrangements.

The features and advantages of the present disclosure will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements.

DETAILED DESCRIPTION

As used herein the expression “semiconductor element” and “semiconductorstructure” are used synonymously and broadly refer to a semiconductormaterial, structure, device, or component of a device. Semiconductorelements include high-quality single crystalline and polycrystallinesemiconductors, semiconductor materials fabricated via high-temperatureprocessing, doped semiconductor materials, organic and inorganicsemiconductors, and composite semiconductor materials and structureshaving one or more additional semiconductor components and/ornon-semiconductor components, such as dielectric layers or materialsand/or conducting layers or materials. Semiconductor elements includesemiconductor devices and device components including, but not limitedto, transistors, photovoltaics including solar cells, diodes,light-emitting diodes, lasers, p-n junctions, photodiodes, integratedcircuits, and sensors. In addition, semiconductor element can refer to apart or portion that forms a functional semiconductor device or product.

“Semiconductor” refers to any material that is an insulator at a verylow temperature, but which has an appreciable electrical conductivity attemperatures of about 300 Kelvin. The electrical characteristics of asemiconductor can be modified by the addition of impurities or dopantsand controlled by the use of electrical fields. In the presentdescription, use of the term semiconductor is intended to be consistentwith use of this term in the art of microelectronics and electronicdevices. Semiconductors useful in the present invention can includeelemental semiconductors, such as silicon, germanium and diamond, andcompound semiconductors, for example group IV compound semiconductorssuch as SiC and SiGe, group III-V semiconductors such as AlSb, AlAs,Aln, AlP, BN, GaSb, GaAs, GaN, GaP, InSb, InAs, InN, and InP, groupIII-V ternary semiconductors alloys such as AlxGal-xAs, group II-VIsemiconductors such as CsSe, CdS, CdTe, ZnO, ZnSe, ZnS, and ZnTe, groupI-VII semiconductors CuCl, group IV-VI semiconductors such as PbS, PbTeand SnS, layer semiconductors such as PbI2, MoS2 and GaSe, oxidesemiconductors such as CuO and Cu2O. The term semiconductor includesintrinsic semiconductors and extrinsic semiconductors that are dopedwith one or more selected materials, including semiconductor havingp-type doping materials and n-type doping materials, to providebeneficial electronic properties useful for a given application ordevice. The term semiconductor includes composite materials comprising amixture of semiconductors and/or dopants. Specific semiconductormaterials useful in some applications of the present invention include,but are not limited to, Si, Ge, SiC, AlP, AlAs, AlSb, GaN, GaP, GaAs,GaSb, InP, InAs, GaSb, InP, InAs, InSb, ZnO, ZnSe, ZnTe, CdS, CdSe,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, PbS, PbSe, PbTe, AlGaAs, AlInAs,AlInP, GaAsP, GaInAs, GaInP, AlGaAsSb, AlGaInP, and GaInAsP. Poroussilicon semiconductor materials are useful for applications of thepresent invention in the field of sensors and light-emitting materials,such as light-emitting diodes (LEDs) and solid-state lasers. Impuritiesof semiconductor materials are atoms, elements, ions or molecules otherthan the semiconductor material(s) themselves or any dopants provided inthe semiconductor material. Impurities are undesirable materials presentin semiconductor materials that can negatively impact the electronicproperties of semiconductor materials, and include but are not limitedto oxygen, carbon, and metals including heavy metals. Heavy-metalimpurities include, but are not limited to, the group of elementsbetween copper and lead on the periodic table, calcium, sodium, and allions, compounds and/or complexes thereof.

“Substrate” refers to a structure or material on which, or in which, aprocess is (or has been) conducted, such as patterning, assembly orintegration of semiconductor elements. Substrates include, but are notlimited to: (i) a structure upon which semiconductor elements arefabricated, deposited, transferred or supported (also referred to as anative substrate); (ii) a device substrate, for example an electronicdevice substrate; (iii) a donor substrate having elements, such assemiconductor elements, for subsequent transfer, assembly orintegration; and (iv) a target substrate for receiving printablestructures, such as semiconductor elements. A donor substrate can be,but is not necessarily, a native substrate.

“Destination substrate” as used herein refers to the target substrate(e.g., non-native substrate) for receiving printable structures, such assemiconductor elements. Examples of destination substrate materialsinclude polymer, plastic, resin, polyimide, polyethylene naphthalate,polyethylene terephthalate, metal, metal foil, glass, flexible glass, asemiconductor, and sapphire.

The terms “micro” and “micro-device” as used herein refer to thedescriptive size of certain devices or structures in accordance withembodiments of the invention. As used herein, the terms “micro” and“micro-device” are meant to refer to structures or devices on the scaleof 0.5 to 250 μm (e.g., from 0.5 to 2 μm, 2 to 5 μm, 5 to 10 μm, 10 to20 μm, 20 to 50 μm, 20 to 50 μm, 50 to 100 μm, or 100 to 250 μm).However, it is to be appreciated that embodiments of the presentinvention are not necessarily so limited, and that certain aspects ofthe embodiments can be applicable to larger or smaller size scales.

“Printable” relates to materials, structures, device components, orintegrated functional devices that are capable of transfer, assembly,patterning, organizing, or integrating onto or into substrates withoutexposure of the substrate to high temperatures (i.e. at temperaturesless than or equal to about 400, 200, or 150 degrees Celsius). In oneembodiment of the present invention, printable materials, elements,device components, or devices are capable of transfer, assembly,patterning, organizing and/or integrating onto or into substrates viasolution printing, micro-transfer printing, or dry transfer contactprinting.

“Printable semiconductor elements” of the present invention comprisesemiconductor structures that can be assembled or integrated ontosubstrate surfaces, for example by using dry transfer contact printing,micro-transfer printing, or solution printing methods. In oneembodiment, printable semiconductor elements of the present inventionare unitary single crystalline, polycrystalline or microcrystallineinorganic semiconductor structures. In the context of this description,a unitary structure is a monolithic element having features that aremechanically connected. Semiconductor elements of the present inventioncan be undoped or doped, can have a selected spatial distribution ofdopants and can be doped with a plurality of different dopant materials,including p- and n-type dopants. The present invention includesmicrostructured printable semiconductor elements having at least onecross-sectional dimension greater than or equal to about 1 micron andnanostructured printable semiconductor elements having at least onecross-sectional dimension less than or equal to about 1 micron.Printable semiconductor elements useful in many applications compriseelements derived from “top down” processing of high-purity bulkmaterials, such as high-purity crystalline semiconductor wafersgenerated using conventional high-temperature processing techniques. Inone embodiment, printable semiconductor elements of the presentinvention comprise composite structures having a semiconductoroperationally connected to at least one additional device component orstructure, such as a conducting layer, dielectric layer, electrode,additional semiconductor structure, or any combination of these. In oneembodiment, printable semiconductor elements of the present inventioncomprise stretchable semiconductor elements or heterogeneoussemiconductor elements.

The term “flexible” refers to the ability of a material, structure,device or device component to be reversibly deformed into a curvedshape, e.g., without undergoing a transformation that introducessignificant strain, such as strain characterizing the failure point of amaterial, structure, device, or device component.

“Plastic” refers to any synthetic or naturally occurring material orcombination of materials that can be molded or shaped, generally whenheated, and hardened into a desired shape. Exemplary plastics useful inthe devices and methods of the present invention include, but are notlimited to, polymers, resins and cellulose derivatives. In the presentdescription, the term plastic is intended to include composite plasticmaterials comprising one or more plastics with one or more additives,such as structural enhancers, fillers, fibers, plasticizers, stabilizersor additives which can provide desired chemical or physical properties.“Dielectric” and “dielectric material” are used synonymously in thepresent description and refer to a substance that is highly resistant toflow of electric current and can be polarized by an applied electricfield. Useful dielectric materials include, but are not limited to,SiO2, Ta2O5, TiO2, ZrO2, Y2O3, SiN4, STO, BST, PLZT, PMN, and PZT.

“Polymer” refers to a molecule comprising a plurality of repeatingchemical groups, typically referred to as monomers. Polymers are oftencharacterized by high molecular masses. Polymers useable in the presentinvention can be organic polymers or inorganic polymers and can be inamorphous, semi-amorphous, crystalline or partially crystalline states.Polymers can comprise monomers having the same chemical composition orcan comprise a plurality of monomers having different chemicalcompositions, such as a copolymer. Cross-linked polymers having linkedmonomer chains are particularly useful for some applications of thepresent invention. Polymers useable in the methods, devices and devicecomponents of the present invention include, but are not limited toplastics, elastomers, thermoplastic elastomers, elastoplastics,thermostats, thermoplastics and acrylates. Exemplary polymers include,but are not limited to acetal polymers, biodegradable polymers,cellulosic polymers, fluoropolymers, nylons, polyacrylonitrile polymers,polyamide-imide polymers, polyimides, polyarylates, polybenzimidazole,polybutylene, polycarbonate, polyesters, polyetherimide, polyethylene,polyethylene copolymers and modified polyethylenes, polyketones,poly(methyl methacrylate, polymethylpentene, polyphenylene oxides andpolyphenylene sulfides, polyphthalamide, polypropylene, polyurethanes,styrenic resins, sulphone based resins, vinyl-based resins or anycombinations of these.

“Micro-transfer printing” as used herein refers to systems, methods, andtechniques for the deterministic assembly of micro- and nano-materials,devices, and semiconductor elements into spatially organized, functionalarrangements with two-dimensional and three-dimensional layouts. It isoften difficult to pick up and place ultra-thin or small devices;however, micro-transfer printing permits the selection and applicationof these ultra-thin, fragile, or small devices, such as micro-LEDs,without causing damage to the devices themselves. Microstructured stamps(e.g., elastomeric, electrostatic stamps, or hybridelastomeric/electrostatic stamps) can be used to pick up micro devices,transport the micro devices to a destination substrate, and print themicro devices onto the destination substrate. In some embodiments,surface adhesion forces are used to control the selection and printingof these devices onto the destination substrate. This process can beperformed massively in parallel. The stamps can be designed to transfera single device or hundreds to thousands of discrete structures in asingle pick-up-and-print operation. For a discussion of micro-transferprinting generally, see U.S. Pat. Nos. 7,622,367 and 8,506,867, each ofwhich is hereby incorporated by reference in its entirety.

The disclosed technology relates generally to systems and methods forcontrolling the release of micro objects (devices) from a nativesubstrate using tethers. In some embodiments, micro objects are designedor configured so that their orientation and spatial configuration arepreserved when the micro objects are released from a substrate. Anchorstructures that remain rigidly attached to the native substrate andtether structures that join the releasable micro object to the anchorserve to maintain the spatial configuration of the micro objects uponpartial or full release of the micro object from the substrate. This canbe accomplished by selective removal of a sacrificial layer (e.g., atleast partially underneath the micro object) by undercutting, etching,ablation, or other means. In some embodiments, the sacrificial layer isa portion of the native substrate on which the active components aregrown. This leaves each micro device supported on and connected to thenative substrate by at least one anchor and at least one tether.

In some embodiments, the etching process to remove the sacrificial layerunderneath the releasable micro object has a crystallographicdependence, etching faster in some directions of a crystal structure andslower in other directions of the crystal structure. Corner structuresetch at different rates because of differences in the number of danglingbonds that are susceptible to different etch rates. For example, in aplanar crystal structure, a crystal atom or molecule that is normallyconnected to four neighbors will only be connected to two neighbors at aconvex corner but will be connected to three neighbors at a concavecorner. A crystal atom normally connected to eight neighbors will onlybe connected to three neighbors at a convex corner but will be connectedto seven neighbors at a concave corner. Therefore, convex or exteriorcorners of structures made of the sacrificial layer etch relativelyquickly, progressively etching and producing etch fronts parallel to thefast etching planes in the crystal. Concave or internal corners of thestructures made of the sacrificial layer have fewer susceptible danglingbonds and etch more slowly, forming a slowly moving or pinned/stoppedetch front defined by the slowly etching planes, provided that theresulting etch fronts of the etchants can form and maintain a localshape characterized by the internal/concave corners. Some release layers(also referred to as a sacrificial layer) that exhibit this kind ofcrystallographic selectivity include Si (1 1 1), Si (1 0 0), InAlP, InP,GaAs, InGaAs, AlGaAs, GaSb, GaAlSb, AlSb, InSb, InGaAlSbAs, InAlSb, andInGaP.

It is often convenient to provide a sacrificial layer that at leasttemporarily spans the entire area of the first substrate, both in theregions used to form releasable micro objects and in the regions used toform anchoring structures. In those applications, anchoring designs canbe used in which the anchor forms a continuous structure, spanning morethan one releasable micro object in at least one dimension or designs inwhich the anchor is positioned about the releasable micro objects. Insome embodiments, the anchoring structures are designed with locallyconcave or internal corners and the releasable micro objects aredesigned with convex/external corners. In some embodiments, thesacrificial material removal etch progresses rapidly under thereleasable micro object (e.g., relative to the rate at which the anchorstructure is formed). These designs can be further benefited by tetherdesigns that include fewer locally concave or internal corners, forexample, by having a narrow shape that is narrower than the anchorstructure to which the tether is attached. In some embodiments, narrowtethers means tethers with a width of 1 μm to 100 μm, 1 μm to 5 μm, 5 μmto 10 μm, 10 μm to 15 μm, 15 μm to 20 μm, or 20 μm to 40 μm. In someembodiments, the width of narrow tethers is 1 μm to 50 μm of 5 μm to 10μm.

FIG. 1 is an illustration of an example native substrate 102 with microdevices (e.g., 106 a, 106 b, and 106 c) formed in an active layer 106thereon. The native substrate 102 will vary depending on the type ofmicro devices formed thereon. Example native substrates includesemiconductor materials (e.g., an amorphous, polycrystalline,microcrystalline, or crystalline semiconductor), metal, polymer, orglass. The active layer 106 (including the active components 106 a-106c) can include a semiconductor, such as crystalline Si, GaAs, GaN, or aIII-V compound semiconductor.

A sacrificial layer 104 is formed on the native substrate 102. Activecomponents 106 can be formed in the active layer 106 formed on thesacrificial layer 104. The active components 106 a-106 c are distributedover the native substrate 102 and are spatially separated from eachother by anchors 108 a-108 b (collectively anchors 108). The anchors 108are physically connected or secured to the native substrate 102 byportions of the sacrificial layer 104 a, 104 b. The active components106 a-106 c are suspended above the native substrate 102 by tethers 110a-110 d (collectively tethers 110) that physically secure each activecomponent 106 a-106 d to one or more anchors 108. In some embodiments, asingle anchor or tether (not shown) is used to secure each activecomponent 106 a-106 c to the substrate 102. The tethers 110 and anchors108 can be at least in part electrically conductive thereby electricallyconnecting each active component 106 a-106 c to one or more tethers 110or anchors 108. In some embodiments, electrically functional anchors 108include passive electrical elements such as electrical conductors,resistors, capacitors, or contact pads, or active components such astransistors and capacitors.

The anchors 108 are connected to the substrate layer 102 by thesacrificial layer 104 beneath the electrically functional anchors 108.The sacrificial layer 104, in some embodiments, is absent in the areas112 a-112 c (collectively 112) below the active components 106 a-106 d,thereby isolating the active components 106 a-106 d from the anchors108. In some embodiments, the sacrificial layer 104 is absent in areas112 below the active components 106 a-106 d after an etching process hasbeen performed to remove the sacrificial layer 104 from those areas. Forexample, in some embodiments, transferable semiconductor structures aregrown on a native substrate material, such as Si (1 0 0). The structurescan be released by undercut etching the substrate materials or asacrificial material formed between the native substrate material andthe transferable semiconductor structures.

In some embodiments, the tethers 110 or anchors 108 are heterogeneoussuch that they are made of a material other than the base substratematerial (e.g., a material other than the semiconductor material thatforms the native substrate). For example, the tethers 110 or anchors 108can be crystalline, polycrystalline, amorphous, or ductile.

The sacrificial layer 104 can be an oxide, such as silicon dioxide. Thetethers 110 can include metal, semiconductors, or doped semiconductors,and/or metal layers thereby providing electrical conductivity betweenthe anchors 108 and the printable active components 106 a-106 d. Thetethers 110 or anchors 108 can include non-conductive dielectrics orresins.

FIG. 2 illustrates the process of undercut etching the sacrificial layerover time. The dotted lines in FIG. 2 indicate the extent of the etchantbeneath the semiconductor device 202 (corresponding to the activecomponents 106 a-106 d). The semiconductor device 202 is shown at t=0before the etching process to release the semiconductor structure 202has begun. In some embodiments, anchor structures 108 are formedperpendicular to the (1 2 2) family of directions of crystalline silicon111. In some embodiments, the anchor structures 108 form structures thatare at least twice as wide as the device 202 to be released. The anchorstructures 108, in some embodiments, encircle the device 202 structureto be released so that the anchor structures 108 are not released by theundercut etch. In some embodiments, orientation of the sacrificial layer104 is selected to allow the undercut etch progression to occuraccording to FIG. 2, as shown from t=0 to t=6 with a vertical fast-etchdirection corresponding to the progression of the dotted-line etch frontunder the device 202. In some embodiments, the sacrificial layer 104 isof sufficient thickness to allow the etchant to flow within thesacrificial layer 104 once the sacrificial material is removed.

In some embodiments, the device 202 structure to be released holdsresidual stress, for example due to epitaxial growth of active materials(e.g. active layer 106) or buffers. Upon release by undercut etching,the device 202 structure can partially or fully relax, deform, or moveand therefore apply additional stress on the portions of the structurestill attached to the substrate (e.g., the tethers 204 corresponding tothe tethers 110 of FIG. 1). In some embodiments, if the stress appliedto the tethers 204 is great enough to fracture the tethers 204, then thedevice 202 structure can be prematurely lost in the release chemistrybefore the micro devices 202 are employed in the micro-printing process(e.g., picked up by a conformable transfer element).

FIG. 3 is an illustration of the process of undercut etching atransferable semiconductor 302 (corresponding to the active components106 a-106 d) with a single tether 304 (corresponding to a tether 110 ofFIG. 1). Undercut etch progression beneath the semiconductor devices 302occurs according to the progression shown in FIG. 3 with the dottedlines. In some embodiments, the device 302 structure to be releasedholds residual stress, for example, due to epitaxial growth of activematerials (e.g. active layer 106) or buffers. Upon release by undercutetching, the device 302 structure can partially or fully relax, deform,or move, and therefore apply additional stress on the portions of thestructure still attached to the substrate (e.g., tether 304corresponding to the tethers 110 of FIG. 1). If the device 302 structureis held by a single tether 304 located at the center point of ananchor-facing edge device 302 structure, the undercut completes along aline at the center of the structure, and free expansion or contractionof the non-tethered edge can accommodate the deformation that occursupon release without applying stress to the tether 304.

FIG. 4 is an illustration of the process of undercut etching atransferable semiconductor device 402 (corresponding to the activecomponents 106 a-106 d) with a single tether 404 (corresponding to thetethers 110 of FIG. 1) placed off-center relative to the edge of thedevice 402, for example by at least 5%, 10%, 20%, 50% or more. In someembodiments, the single tether 404 of a releasable structure, forexample, on Si (1 0 0), is placed off-center 406 with respect to theedge of the device 402 on the anchor-facing edge 408 e.g., the tether404 is not located at the mid-point or along a center line of the device402. The undercut etch progression reaches completion with the areas 410near the tether 404 releasing last. In this configuration, the designoffers, among other things, added surety that the relaxation of residualstress in the releasable structure is accommodated by freeexpansion/contraction of the non-tethered edges and does not break thetether 404 and cause yield loss during the release process. Off-centertethers 404, in some embodiments, are more likely to finish with lessunreleased regions under a device 402 (e.g., perfectly or nearperfectly). As shown in the illustrations in FIG. 4, there is no arrest(stopping) of the etch planes due to the location of the tether 404 inan off-center position.

FIGS. 5A-B are illustrations of an example structure with four tethers502A-502D (collectively tethers 502 corresponding to the tethers 110 ofFIG. 1) per releasable structure 504 (corresponding to the activecomponents 106 a-106 d). In some embodiments of producing releasablestructures 504 on substrates 102 (shown in FIG. 1), such as Si (1 0 0),tethers 502A-D are off-center and are placed at locations on theperimeter of the chip 504 to arrest the etch front 506 by producingconcave vertices at the intersection of the slow-etching planes beforethe structure is released. In some embodiments, pinning from theslow-etching plane is avoided.

In some embodiments, the tethers 502 are designed (e.g., in theirplacement) such that they do not support concave vertices at theintersection of slow-etching planes, thereby avoiding the arrest of theetch front. Such tethers 502 can, for example, have an aspect ratio of1.732, for example, for (1 1 1) Silicon. In some embodiments, the aspectratio is from 1.3 to 1.5, 1.5 to 1.7, 1.7 to 1.9, or 1.9 to 2.1. In someembodiments, the critical aspect ratio of the tether 502 is dependent onthe crystal orientation of the etched material. The critical aspectratio can be determined empirically by etching tethers of differentaspect ratios to determine a useful aspect ratio or calculated bycomparing the etch rates in the different directions under the preferredetch conditions.

FIGS. 6A and 6B are example illustrations of tethers specific to (1 1 1)Silicon. FIG. 18 is a prior-art illustration of crystal orientation forsemiconductor devices. The aspect ratio of a tether 602 (correspondingto the tethers 110 of FIG. 1) can be set to avoid pinning fromslow-etching planes. Different crystals can have different criticalaspect ratios. The tethers 602 shown in FIGS. 6A and 6B have twodifferent aspect ratios since the width of the tether 602 in FIG. 6A isless than the width of the tether 602 in FIG. 6B and the aspect ratio isdefined by dividing the width by the height of the tether 602. Thetether 602 shown in FIG. 6A has an aspect ratio less than 1.732, and thetether shown in FIG. 6B has an aspect ratio greater than 1.732,respectively. As shown in FIG. 6A, a tether 602 with an aspect ratioless than 1.732 can form vertices (the facing corners of the trianglesdelineated by dotted lines) that arrest the undercut. In contrast, asshown in FIG. 6B, a tether 604 with an aspect ratio greater than 1.732does not arrest undercut.

FIGS. 7A and 7B illustrate additional designs for crystallographicselectivity in removing a sacrificial layer 702 a and 702 b(collectively 702) from underneath the releasable objects 704 a-704 d(collectively 704 and e.g., such as active components 106 a-106 c ofFIG. 1) while reducing the amount of the sacrificial layer 702 removedfrom underneath the anchoring structure. In micro assembly, in certainembodiments, it is useful to release micro objects 704 a-704 ddelineated, for example, by trenches 706 a-706 d (collectively 706) froma first substrate while preserving their orientation and spatialconfiguration until they are selected (e.g., picked up) by a conformabletransfer element, such as an elastomer stamp. Anchor structures thatremain rigidly attached to the first substrate and tether structures 708a-708 h (collectively 708) that join the releasable micro object to theanchor can serve to maintain the spatial configuration of the microobjects upon the partial or full release of the micro objects 704 fromthe first substrate. For example, this can be accomplished by theselective removal of a sacrificial layer 702 by etching or ablation orother means. In this example, object 704 a is secured to an anchorstructure with a single tether 708 a, object 704 b is secured to ananchor structure with two tethers 708 b and 708 c, object 704 c issecured to an anchor structure with four tethers 708 d, 708 e, 708 f,and 708 g, and object 704 d is secured to an anchor structure with twotethers 708 h and 708 i. In some embodiments, each object 704 on anative-substrate is secured with the same number of tethers.

In some embodiments, the etching process to remove the sacrificial layerunderneath the releasable micro object has crystallographic dependencein which etching occurs faster in some directions of the crystalstructure and slower in other directions of the crystal structure. Inthose cases, convex or exterior corners of structures (e.g., of thesacrificial layer) etch quicker to progressively produce etch frontsthat are parallel to the fast etching planes, as described above. Insuch instances, concave or internal corners of the structures (e.g., ofthe sacrificial layer) etch slower, thereby forming a slowly moving orpinned/stopped etch front, which is defined by the slowly etching planes(provided that the resulting etch fronts of the etchants can form andmaintain a local shape characterized by the internal/concave corners).Certain release layers (e.g., sacrificial layer) having crystallographicdependence that exhibit this kind of crystallographic selectivityinclude Si (1 1 1), Si (1 0 0), InAlP, InP, GaAs, InGaAs, AlGaAs, GaSb,GaAlSb, AlSb, InSb, InGaAlSbAs, InAlSb, and InGaP.

Another release layer that exhibits this kind of crystallographicselectivity is Si (1 0 0). In some embodiments, the substrate is a Siwafer with a first surface parallel to the (1 0 0) plane. In theseembodiments, heated aqueous bases, such as KOH, NaOH, or TMAH solutions,remove silicon below micro devices. The crystallographic selectivity ofthe etch process leaves the regions of silicon below anchoringstructures unetched or mostly unetched. In these embodiments, theanchoring structures and the micro devices effectively withstandexposure to the heated aqueous bases. In certain embodiments, if thecrystal structure of the anchoring structures and the micro devices isdifferent and/or oriented in a particular way, they will not etchreadily. Thus, the sacrificial layer can be etched without (e.g.,minimally) etching other structures, such as the anchoring structures.Additionally, the in-plane geometry of the anchoring structures and themicro devices determine which regions of the Si (1 0 0) are etched andwhich regions survive during the exposure to the heated aqueous base.Regions of Si (1 0 0) that are uncovered by an etch mask are etched inthe [1 0 0] direction. Anchoring structures that are parallel to the <01 1> directions and are long and/or have only internal (concave) cornersdo not undercut but instead protect the regions of silicon underneaththe anchoring structures, forming tapered sidewalls perpendicular to the{1 1 1} planes as the etch front progressed downward, thereby forminganchors that persist through an undercut release operation. Thecrystallographically selective etchants undercut convex corners(external corners), thereby releasing printable objects for microassembly.

In some embodiments, the anchoring structures are continuous lines thatrun parallel to the <0 1 1> directions. In some embodiments, theprintable structures are tethered to the anchors by a single tetheringstructure designed to fracture upon retrieval by a transfer element, forexample an elastomer stamp. In some embodiments, printable structuresare tethered to two anchoring structures that are not aligned in a lineparallel to the <0 1 1> directions to avoid pinning of the etch front bythe formation of internal corners that do not undercut. In someembodiments, printable structures are tethered by three or more tetherswith each tether connecting a printable structure and one of twoanchoring structures on opposing sides of the printable structure. Incertain embodiments, no two tethers are aligned in a line parallel tothe <0 1 1> direction that spans from one anchoring structure to theother anchoring structure on opposing sides of the printable structure,thereby avoiding unwanted pinning of the etch front during the releaseprocess. In some embodiments, the tether structures and the <0 1 1>direction form an oblique angle. In some embodiments, the tethers, theprintable structure, and the anchors have geometries such that norectangle with edges parallel to the <0 1 1> directions that has an edgepassing through the printable object may be drawn such that all portionsof the rectangle are covered by the anchors, tethers, and printablestructure.

Referring to the cross section of FIG. 21A, a native substrate 102 hasan active component 106 formed thereon together with anchors 108 andtethers 110. The native substrate 102 is etched to form the cavity area112 as shown in FIG. 21B, enabling the printable active component 106 tobe released from the native substrate 102 and micro transfer printed toa different, non-native substrate. The silicon crystal orientation isreferenced in the figure.

FIGS. 22 and 23 illustrate geometries of Si 1 0 0 that will producereleasable and micro transfer printable active components 106. The planview of FIG. 22 illustrates the same structure as in FIGS. 21A and 21Bwith a single offset tether 108 to the printable micro-device 106 on theleft side and double offset tethers 108 to the printable activecomponent 106 on the right side. Offset tethers are offset with respectto a side of the printable active component 106. The plan view of FIG.23 also illustrates the same structure as in FIGS. 21A and 21B withtriple tethers 108 to the printable active component 106 on the leftside (at least one, two, or all of which are offset) and quadrupleoffset tethers 108 to the printable active component 106 on the rightside (at least one, two, three, or all of which are offset).

FIG. 24 illustrates tethers 108 that have edges that are non-orthogonalto edges of the printable active component 106 or anchor and that arenon-orthogonal to the crystalline direction [0 1 1]. Furthermore, in anembodiment, the portion of the tether 110 in contact with the anchor 110is not between the portion of the tether 110 in contact with theprintable active component 106 in the y-direction, for example separatedby a distance 109, as shown in FIG. 24. Mathematically, in thisembodiment the releasable micro object 106 is separated from the anchor108 in the x-direction by a separation distance x, the tether 110 isattached to the anchor 108 or the releasable micro object 106 at anacute angle theta, the distance between the first edge and second edgeof the tether at which the tether 110 is attached to the anchor 108 orreleasable micro object 106 is y, and y is less than or equal tox*tan(theta). Thus, x is the gap between the anchor 108 and thereleasable micro object 106, theta is the angle of the tether 110 withrespect to the anchor 108 or the releasable micro object 106 (with anangle of zero indicating that the tether 110 is parallel to the edge ofthe anchor 108 or the releasable micro object 106), and y is the widthof the tether 110 in a direction parallel to the edge of the anchor 108or parallel to the edge of the releasable micro object 106. FIG. 26illustrates an embodiment with similar tethers 110 attached to thecorners of the releasable micro object 106. These structures releasemore readily because etch fronts from neighboring ingress points oneither side of the tether 110 will overlap and produce an etch maskstructure with all convex (exterior) corners and no concave corners.

Thus, in a releasable embodiment, the tether 110 has opposing sides thatare not attached to the anchor 108 or the releasable micro object 106and the opposing sides overlap in at least one of the y-direction andthe x-direction.

In contrast to the embodiments of FIGS. 22-24, FIG. 25 illustratesarrangements of tethers 108, anchors 110, and printable active component106 that do not form an effective release structure because the etchfronts are pinned. In these cases, similar tethers 108 are not offset.As shown in FIG. 25, the portion of the tether 108 in contact with theanchor 110 overlaps the portion of the tether 108 in contact with theprintable active component 106 in the y-direction, for exampleoverlapping by a distance 111. FIG. 27 illustrates corner tethers 110that also have an overlapping distance 111. These structures do notrelease as readily because etch fronts from each ingress point above andbelow the tether 110 will terminate by slow-etching planes and allconcave corners before they overlap.

In another releasable embodiment referring to FIG. 28, a first pair oftethers 110 a, 110 b are offset in the y-direction and do not overlap inthe offset direction as shown with distance 109 a and a second pair oftethers 110 c, 110 d are offset in the x-direction and do not overlap inthe offset direction as shown with distance 109 b. This structurereleases because etch fronts from neighboring ingress points willoverlap and produce etch mask structure with all convex (exterior)corners; no concave corners. In contrast, FIG. 29 will have difficultyreleasing because the tethers 110 are not offset and etch fronts fromeach ingress point will terminate by slow-etching planes and all concavecorners before they overlap. In FIG. 30 the pair of tethers are offsetin both the x-direction and y-direction and do not overlap in eitheroffset direction. Thus, the structure can release effectively.

To address this effect, in some embodiments, a sacrificial layer isprovided that at least temporarily spans the entire area of the firstsubstrate, both in the regions used to form releasable micro objects andin the regions used to form anchoring structures. In addition, in thoseapplications, a continuous structure, spanning more than one releasablemicro object in at least one dimension is employed. Alternatively, insome embodiments, the anchor is positioned about the releasable microobjects.

In such embodiments, the anchoring structure is characterized by locallyconcave or internal corners and the releasable micro objects are locallycharacterized by convex or external corners. In such configurations, theetch progresses rapidly (e.g., relative to the rate at which theanchoring structures are released) under the releasable micro object.

In some embodiments, the tethers are designed to not support theestablishment of locally-concave or internal corners. In suchembodiments, the tethers are configured, for example, with a narrowshape (e.g., a width of from 10 nm to 100 nm, 100 nm to 500 nm, 500 nmto 1 micron, or 1 micron to 10 microns).

In certain embodiments, the goal is to avoid forming geometries of thetech-blocking materials (i.e., anchors, tethers, and printablestructures) that arrest the etch process. For example, for certaingeometries of the tech-blocking materials, it is not possible to draw arectangle covered by the anchors, tethers, and micro object with an edgeparallel to the <0 1 1> directions that has an edge passing through themicro object.

In certain embodiments, the tethers are formed on a different plane thanthe top surface of the release layer (e.g., silicon or other releaselayer on the substrate). Specifically, the tethers in this embodimentforms air bridges over the release layer such that the tethers do notarrest the etch process regardless of the plan view layout of thetethers.

For example, in certain embodiments, anchor structures formed in directcontact with a release later that has anisotropic etch characteristics(e.g., Si 100, Si 111, InAlP, or InAlGaAs). A printable device formed indirect contact with the release layer. Tether structures (i.e., that canbe fractured during printing) physically connect the printable device tothe anchors, but are at least partly disconnected from the release layer(like air-bridges) such that one may outline an imaginary polygonentirely on the exposed surface of the release layer that encircles theprintable device with the tethers forming air bridges over thatimaginary polygon. In that way, the etch blocking structures arecharacterized by the printable device and not the tethers. The resultingetch mask is characterized by convex corners (exterior corners) and willnot arrest the etching of the release layer. The imaginary polygon canbe described as a convex hull, wherein convex hull means a polygon drawnaround a surface structure that has no concave corners—like a rubberband around the surface structure.

FIGS. 31A and 31B illustrate various embodiments of tether structures3102 and anchor structures 3104 in which the tethers 3102 are at leastpartly disconnected from the release layer 3106. The release layer 3106in these illustrations is on the substrate 3108, although in someembodiments the release layer 3106 is a portion of the substrate 3108itself. The printable devices 3112 are formed on the release layer 3106.The tethers 3102 connect the devices 3112 to the anchors 3104 such thatthe spatial orientation of the devices 3112 is maintained by the anchors3104 and tethers 3102 when the release layer 3106 is removed. As shownin FIGS. 31A and 31B, the tethers 3102 are arranged such that a gap 3110(e.g., air gap) exists between the tethers 3102 and the release layer3106.

To promote a controlled release of the printable micro devices from thefabrication substrate, the tether can be shaped to provide a consistentpoint of separation. FIG. 8 is an illustration of an example notch 802implemented in a tether 804 corresponding to the tethers 110 of FIG. 1.Notches 802 in tethering structures can be used to facilitatecontrolling the point of fracture when a released micro object isretrieved by a transfer element such as a structured elastomer stamp.The width of the notch 802 can be used to control the force required tofracture a tether 802 when a released micro object (e.g., partiallyreleased) is removed by a transfer element.

In another aspect of the disclosure, the disclosed technology enablesthe fabrication and manufacturing of ultra-thin low-cost packagedmicro-scale devices using (1 0 0) silicon. FIG. 9 is a flowchart of anexample method 900 for providing ultra-thin low-cost packagedmicro-scale devices using (1 0 0) silicon. Transfer-ready micro-scaledevices, such as micro-scale integrated circuits or LEDs, are firstfabricated (step 902). The micro-scale devices are assembled, forexample, onto a (1 0 0) silicon wafer with a dielectric layer (step904). In some embodiments, other types of wafers can be used as adestination substrate for the micro devices, for example glass, plastic,or metal. In some embodiments, thin-film metallization processes can beused to interconnect the assembled micro devices with other functionalelements (step 906). The assembled micro-scale devices and metallizationlayer can be fully embedded within a dielectric layer (step 908). Thedielectric layer can be patterned (step 910) and the packagedmicro-scale devices can be released from the underlying wafer (step 912)and transferred to another substrate. Various processes can be used torelease the micro-scale devices from the underlying wafer such as thosedisclosed herein (e.g., see above).

FIGS. 10A through 10D illustrate a process for making transfer-readyintegrated circuits using a silicon-on-insulator wafer. In someembodiments, a (1 0 0) handle wafer is employed. As shown in FIG. 10A,the source substrate, in this example, is a silicon-on-insulator wafer1002. In some embodiments, other types of wafers such as semiconductoror metal wafers can be used. The starting wafer includes a handle waferwith (1 0 0) orientation. A buried oxide layer 1004 (e.g., ˜1 μm thick)is directly on a top surface of the source substrate 1006. A devicesilicon layer 1008 is provided directly on the top surface of the buriedoxide layer 1004.

As shown in FIG. 10B, one or more integrated circuits 1010 (collectivelydevices 1010) are fabricated (directly or indirectly) on the devicesilicon layer 1008. The device silicon layer 1008 can be thick enough tobehave like bulk silicon, can be partially depleted, or can be fullydepleted and can have, for example, a thickness of less than a micron,one micron, or a few microns (e.g. 2 microns), 10 microns, 50 microns,100 microns, or 1000 microns. The process can include providing multiplewiring levels. In some embodiments, a terminal inorganic dielectricpassivation layer is provided such that none of the wiring metallizationof device silicon is exposed.

As shown in FIG. 10C, the micro device is defined by patterning andetching trenches 1014 through the dielectric layers around the perimeterof the device 1010. The device silicon layer is also patterned. If thereis device silicon exposed at the perimeter of the device, an inorganicdielectric layer can be deposited to passivate this exposed silicon. Thepassivation layer is then patterned forming inorganic dielectricportions 1012 a, 1012 b, and 1012 c. In some embodiments, the silicon isetched to an appropriate depth 1016 to release the device 1010 (FIG.10D). As shown in FIG. 10D, the device 1010 is released from the sourcesubstrate 1006 by undercutting a sacrificial layer 1018 using techniquessuch as those disclosed herein (e.g., as described above). Afterperforming the undercutting process, the devices 1010 are held in placeby anchors and/or tethers (not shown in FIG. 10D) as described above.

FIGS. 11A through 11C illustrate an example process of assembling alow-cost carrier wafer using micro-assembly techniques usingelastomer-stamp micro-transfer-printing to pick up and place the thinmicro-scale devices onto another carrier wafer. FIG. 11A illustrates atransfer-stamp 1102 as it approaches one or more transfer-readymicro-scale devices (e.g., integrated circuits) 1010 on a source wafer1006. FIG. 11B illustrates the transfer-stamp 1102 picking up thedevices 1010 from the source wafer 1006. The devices 1010 aretransferred to a low-cost carrier wafer 1112 as shown in FIG. 11C. Insome embodiments, a layer 1106 is formed on the carrier wafer 1112, suchas an insulation layer or polymer layer (e.g., a tacky polymer layer).Example transfer techniques are described in U.S. Pat. Nos. 7,622,367,7,943,491, and 8,506,867, each of which is hereby incorporated byreference.

FIGS. 12A through 12C illustrate an example method for generating thinand low-cost packaged micro-scale devices using the (1 0 0) system. Asshown in FIG. 12A, micro-scale devices 1010 a and 1010 b (collectively1010) are assembled onto the carrier wafer 1112 using micro assemblytechniques. The carrier wafer 1112, in some embodiments, is (1 0 0)oriented silicon and includes a dielectric layer 1206. The dielectriclayer 1206 can be organic (e.g., polyimide) or inorganic (e.g., silicondioxide). In some embodiments, the assembled micro-scale devices 1010are embedded within a second layer 1202 of dielectric as shown in FIG.12B. This second embedding dielectric 1202 can be organic (e.g., a spincoated polyimide) or can be a deposited inorganic material. Referring toFIG. 12C, the dielectric layers 1202 and 1206 can be then patterned in amanner that defines the desired micro-scale device packages 1204 a and1204 b that each include a micro-scale device 1010 and also defines theanchors 108 a-108 c and tethers 110 a-110 d (as described above). Insome embodiments, the micro-scale device 1010 packages are then releasedfrom the wafer 1102 using the undercut removal process as describedabove, for example. The resulting wafer-level thin and low-costmicro-scale device 1010 packages are suitable for transfer to othersubstrates using, for example, vacuum collets. In some embodiments, padstructures are formed on top or bottom surfaces of the embeddedmicro-scale devices 1010, thereby forming a surface-mountable device.

FIG. 13 is an illustration of an example destination substrate withmicro-scale devices printed into a structure. The destination substratecan be made of an inorganic material, such as silicon dioxide, glass, ora metal. Printed micro-scale devices can be embedded within thestructures as shown in FIG. 13.

FIG. 14 is an illustration of example adaptable assembly processes withwafer-level (1 0 0) silicon-enabled packages. Micro-scale devices on apackaged wafer 1402 can be transfer printed to a tape-and-reel 1404. Thetape-and-reel 1404 can be provided to a tape-fed high-speed chip shooter1406 for assembly. The tape-fed high-speed chip shooter 1406 can packagethe micro-scale devices on the tape-and-reel 1404 as, for example, apackage-in-package molded package 1408 or a package-on-lead-frame thatis subsequently molded into a package 1410.

In some embodiments, the package wafer 1402 is provided to a wafer-feddie-attach tool 1412. The wafer-fed die-attach tool 1412 can package themicro-scale devices into wafer-level-packages 1402. The wafer-feddie-attach tool 1412 can package the micro-scale devices onpackage-on-lead frames that are subsequently molded into a package 1410.The wafer-fed die attach tool 1412 can package the micro-scale devicesinto package-in-package devices 1408 or into a wafer-level package (WLP)1414.

In some embodiments, the package wafer is provided to a wafer-fedmicro-transfer printer 1416 which packages the micro-scale devices inwafer-level-packages 1414.

FIG. 15 is an illustration of an example system 1504 in a package usingthe (1 0 0) release system. The illustrated example device 150 caninclude a small integrated circuit 1514 that is interconnected with twodifferent types of sensors 1508, 1510 and also interconnected with anantenna 1512 made using a common wafer-level metallization. This systemcan be entirely embedded in a dielectric material and can be releasedfrom the underlying (1 0 0) wafer as described above. The device 1504can be coupled to anchors 1506 a and 1506 b by tethers 1502 a-1502 d.This provides a small package that can be assembled onto othersubstrates using, for example, micro-assembly techniques.

In another aspect of the disclosure, the anchor 110 and tether 108 areformed of a single polymer layer. The polymer layer further serves asprotection for the fabricated micro-scale devices by encapsulating themicro-scale devices after they are fabricated on the native substrateand during the micro-printing process. To this end, the encapsulationserves to protect any chemical-sensitive layers within fabricated microdevices. The encapsulating polymer layer, for example, provideselectrical insulation and protection from contaminants, moisture, andoxidizers. The encapsulating polymer layer further allows forelectrically contactable structures to be employed on the bottom side,top side, or lateral side of a printable object.

The polymer layer, in some embodiments, can be made of a photoresist orphoto-sensitive material with sufficient stiffness to maintain thespatial configurations of printable structures during a release process.The polymer that forms the anchor, tether, or encapsulation themicro-scale device can also be selectively removed from the printableobjects and the target substrate after transfer by dissolution, etchingor other processes. The polymer can have sufficient adhesion to serve asa contact interface with a transfer element of the micro-printing systemso that it can be picked up from a native substrate and can be treatedto serve as an interface for separation between the transfer element andthe micro-scale devices once the micro-scale devices have beentransferred to a destination substrate.

In some embodiments, the single polymer layer improves the density ofthe printable, micro devices for a given wafer. The supporting,non-printable structures (e.g., the anchor and tether) can be fabricatedsuch that the distance between adjacent printable structures isminiaturized, for example less than 500 microns, less than 100 microns,less than 50 microns, less than 25 microns, or less than 10 microns.

FIG. 16 illustrates an embodiment of a releasable micro object 106comprising anchors 108, tethers (not shown) and encapsulating layer 1602formed of a single polymer layer 1601. As shown in the figure, thereleasable micro object 106 is a printable active component having anemitter 1604, a collector 1606, a sub-collector 1608, and electricalpads 1610. The releasable micro object 106 is fabricated over asacrificial layer 104, referred to in the figure as a release layer.

The polymer layer 1601 is applied over the releasable micro object 106and over the native source substrate 102. As shown, the polymer layer1601 encapsulates the exposed top and side regions of the micro object106. The polymer layer 1601 also encapsulates a portion of thesacrificial layer 104.

To separate the releasable micro objects 106 from the native substrate102, the polymer layer 1601 is treated to form the anchor structure 108on the source substrate 102 and the tether structure 110 (not shown). Asillustrated in the figure, the anchor structures 108 are formed over thenative substrate 102 (or a substantial portion thereof) to remainrigidly attached to the substrate 102 as the sacrificial material withinthe sacrificial layer 104 is fully or partially removed. The tether 110(not shown) connects the anchor structure 108 to the printable microobject 106 and the encapsulating layer 1602 of the polymer layer 1601.The treatment, in some embodiments, also forms ports of ingress (notshown in FIG. 16) in the polymer layer 1601 that exposes a portion ofthe sacrificial layer 104. The ports of ingress 1612 (FIG. 17) allowsaccess to the sacrificial layer 104 to, for example, perform theundercutting removal process (e.g., by chemical etching).

In some embodiments, the treatment includes forming a photo-sensitivelayer (as the polymer layer) on the releasable micro objects 106, thenselectively exposing portions of the photo-sensitive layer to anenergized beam (e.g., electromagnetic-radiation beam or electron beam)to alter solubility of the photo-sensitive layer to a photoresistdeveloper. The photo-sensitive layer (or the entire device) can beexposed to heat to solidify the photo-sensitive layer. Subsequently, theportions of the portions of the photo-sensitive layer (e.g., soluble orinsoluble to the photoresist developer) are then removed to definebreakable tethers.

Photodefinable materials (such as photoresists or cross-linkable resins)can be used for making anchoring, tethering, or encapsulatingstructures, offering ease of formation, and in many cases ease ofremoval by dissolution in wet chemicals, organic solvents, or aqueousmixtures, or by ashing in oxygen and/or fluorine compounds.

The undercutting removal process can employ chemical reactants to etchthe sacrificial layer 104 (shown as the release layer in FIG. 16),gaining access through ports of ingress formed in the anchoring,tethering, and/or encapsulation structures. Etch reaction products canexit the space under the releasable micro objects through ports ofegress. In some embodiments, the ports of egress are the same as theports of ingress (e.g, 1612 in FIG. 17). When the release layer is fullyor partially removed, the releasable micro object is said to be at leastpartially released and ready for removal by a transfer element. Thetransfer element contacts and adheres to the partially released object.In some embodiments, it is advantageous for the transfer element to beconformable to make intimate contact with at least a portion of thetopography of the releasable object. For example, the transfer elementcan include a conformable material, such as PDMS.

FIG. 17 is an illustration of release and removal of example releasablemicro objects 106. As shown, the releasable micro objects 106 are incontact with a transferring element 1702. In some embodiments multipleposts 1704 embodiments contact the releasable micro object 106. In otherembodiments a single post contacts a releasable micro object 106. Afteradhering to the released object 106, the transferring element 1702 movesaway from the native substrate 102 while maintaining adhesion to thereleased micro object 106, thereby retrieving, removing, separating, orpicking the object 106 from its native substrate 102. In the process ofremoval, the anchor 108 or tether 110 structures fracture or otherwiselose connection to the native substrate 102 or the released object 106.In some embodiments, the tether 110 fractures upon contact by thetransferring element 1702. In certain embodiments, the tether 110fractures when the transferring element 1702 pulls the micro object 106from the native substrate 102.

Materials selected for specific fracture properties, adhesionproperties, or definition of geometries with stress concentrationfeatures in the anchoring and tethering structures are beneficial forcontrolling the points of separation or fracture. In some embodiments,the tether 110 is formed of a narrow structure (or shaped with a notchin certain embodiments) to provide a point of fracture and allow theprintable micro object 106 to separate from the native structure 102. Asnoted above, tethers 110 can be made of organic materials, such aspolymers, or inorganic materials such as silicon, silicon dioxide, orsilicon nitride.

Referring still to FIG. 17, the polymer layer 1601, in particular, theencapsulation region 1602, serves as the point of contact with thetransferring element 1702. In other embodiments, the transferringelement 1702 contacts the micro object 106 itself.

As illustrated in FIG. 19, in one embodiment, a tether design includestethers 1902 that are oriented in such a way that the slow-etchingplanes are not allowed to arrest the lateral undercut. The tethers 1902separate the trenches 1908 and connected to the chipset 1904 and anchors1906. A key parameter, as illustrated in FIG. 20 is to ensure that d>0so that the tether 1902 is angled. Such a design is useful, for examplewith InAlP release layers.

In some embodiments, the transferring element 1702 includes one or moreposts, for example, organized in an array, to make contact with a givenprintable micro object 106. In certain embodiments, the transferringelement 1702 forms a single surface to make contact with the polymerlayer 1601.

In view of the structure, functions and apparatus of the systems andmethods described here, in some embodiments, a system and method forproviding micro-scale devices are provided. Having described certainembodiments of methods and apparatus for providing micro-scale devices,it will now become apparent to one of skill in the art that otherembodiments incorporating the concepts of the disclosure can be used.Therefore, the disclosure should not be limited to certain embodiments,but rather should be limited only by the spirit and scope of thefollowing claims.

Throughout the description, where apparatus and systems are described ashaving, including, or comprising specific components, or where processesand methods are described as having, including, or comprising specificsteps, it is contemplated that, additionally, there are apparatus, andsystems of the disclosed technology that consist essentially of, orconsist of, the recited components, and that there are processes andmethods according to the disclosed technology that consist essentiallyof, or consist of, the recited processing steps.

It should be understood that the order of steps or order for performingcertain action is immaterial so long as the disclosed technology remainsoperable. Moreover, two or more steps or actions can be conductedsimultaneously.

What is claimed:
 1. An array of micro devices, the array comprising: asource substrate comprising silicon (1 0 0) having a process side; asacrificial layer comprising silicon (1 0 0) sacrificial material havingan anisotropic crystal structure on the process side of the sourcesubstrate; a plurality of releasable micro objects formed at least inpart on the sacrificial layer; a plurality of anchor structures locatedon the process side of the source substrate, wherein the anchorstructures remain rigidly attached to the source substrate in theabsence of the sacrificial material; and a plurality of tethers, whereineach tether of the plurality of tethers laterally connects a releasablemicro object of the plurality of releasable micro objects to a portionof one of the anchor structures, each releasable micro object isconnected to an anchor by a single tether, and the tethers are shaped tofracture in response to pressure.
 2. The array of claim 1, wherein eachof the plurality of tethers is sized and shaped to break when acorresponding micro object is contacted by an elastomer stamp for microtransfer printing from the source substrate to a target substrate,different from the source substrate.
 3. The array of claim 1, whereinthe sacrificial material is a portion of the source substrate.
 4. Thearray of claim 1, wherein the anchor structure forms a continuousstructure, spanning more than one releasable micro object of theplurality of releasable micro objects in at least one dimension.
 5. Thearray of claim 1, wherein each of the plurality of anchors arecharacterized by locally concave or internal corners and each of theplurality of releasable micro objects is locally characterized by convexor external corners.
 6. The array of claim 1, wherein each of thetethers comprises one or more notches that provide a point of fracturewhen a respective releasable micro object is moved with respect to theanchor structures.
 7. The array of claim 1, wherein the portion of thetether in contact with the anchor does not overlap the portion of thetether in contact with the releasable micro object in a directionparallel to the edge of the releasable micro object or the anchor towhich the tether is attached.
 8. The array of claim 1, wherein thetether has opposing sides that are not attached to the anchor or thereleasable micro object, wherein the opposing sides overlap in at leastone of the x-direction and the y-direction.
 9. The array of claim 1,wherein the releasable micro object is separated from the anchor in thex-direction by a separation distance x, the tether is attached to theanchor or the releasable micro object at an acute angle theta, and thedistance between the first edge and second edges at which the tether isattached to the anchor or releasable micro object is y, and y is lessthan or equal to x*tan(theta).
 10. The array of claim 1, wherein: afirst pair of first and second tethers are offset the y-direction and donot overlap in the offset direction and a second pair of first andsecond tethers are offset in the x-direction perpendicular to the edgeof the anchor or releasable micro object and do not overlap in theoffset direction; or a pair of first and second tethers are offset inthe y-direction and do not overlap in the offset direction and areoffset in the x-direction and do not overlap in the offset direction.11. An array of micro devices, the array comprising: a source substratehaving a process side; a sacrificial layer comprising silicon (1 0 0)sacrificial material on the process side of the source substrate; aplurality of releasable micro objects formed at least in part on thesacrificial layer; a plurality of anchor structures located on theprocess side of the source substrate, wherein the anchor structuresremain rigidly attached to the source substrate in the absence of thesacrificial material; and a plurality of tethers, wherein each tether ofthe plurality of tethers laterally connects a releasable micro object ofthe plurality of releasable micro objects to a portion of one of theanchor structures, the tethers are shaped to fracture in response topressure, and each tether of the plurality of tethers is located on anoff-center, anchor-facing edge of the respective releasable micro objectof the plurality of releasable micro objects, so that in the absence ofthe sacrificial material the releasable micro objects are capable ofmoving with respect to the anchor structures and the tethers deform whenmechanically stressed.
 12. The array of claim 11, wherein thesacrificial material is a portion of the source substrate.
 13. The arrayof claim 11, wherein it is not possible to draw a rectangle covered bythe anchors, tethers, and micro object with an edge parallel to the <0 11> directions that has an edge passing through the micro object.
 14. Thearray of claim 11, wherein any rectangle covered by the anchors,tethers, and printable structure with edges parallel to the <0 1 1>directions does not have an edge passing through the releasable microobject.
 15. The array of claim 11, wherein it is not possible to draw arectangle covered by the anchors, tethers, and printable micro objectwith edges parallel to the <0 1 1> directions that has an edge passingthrough releasable micro object.
 16. The array of claim 11, wherein thetether is disconnected from the substrate surface in the area of aconvex hull drawn around the exposed surface of the sacrificial layer.